Honeycomb sandwich sheet or panel, based on polypropylene, with a number of central thermoformed films

ABSTRACT

A honeycomb sandwich sheet or panel, based on thermoplastic polypropylene, includes a structure having two flat outer films, at the top and bottom, welded to at least two inner or central thermoformed blister films, repeated in a regular and continuous pattern, wherein the at least two inner thermoformed films are welded to each other.

The present invention relates to a honeycomb sandwich sheet or panel,based on polypropylene, with a number of central thermoformed films, theprocess and apparatus for producing said sheet.

Honeycomb or blister sandwich sheet or panel refers to a honeycombsheet, preferably made of polypropylene, that can have a grammage, i.e.weight per square metre, typically ranging from 200 to 4000 g/m²approximately. This product has specific characteristics such as aconsiderable rigidity and hardness, even in the presence of a goodresilience (i.e. fracture strength). It also has a fill factor, i.e.ratio between the volume of plastic material with respect to the totalvolume occupied by the product, of 30÷50%.

Thanks to this particular honeycomb structure, blister sheet has aparticularly interesting resistance/specific weight ratio; inparticular, this feature is of particular interest and importance in thepackaging field, where there is a specific tendency towardsprogressively reducing the weight of the packaging product, thuseliminating the overpackaging phenomenon.

The current state of the art describes and uses honeycomb or blistersheets or panels which, very schematically, consist in the hot joiningof three different films, of which one (the central film) is athermoformed film. As a result of said thermoforming, the central filmacquires the characteristic honeycomb form, this guaranteeing asignificant increase in the static moment of inertia in the threedirections and therefore increasing the relative resistance modules.

The particular conformation of the central film gives the end productmechanical properties (rigidity, load resistance, etc.) and functionalproperties (lightness, malleability, etc.), that in recent years haveled to the use of a blister panel or sheet in various applications, withexcellent results.

The product, as briefly described above, however, has some significantcritical aspects, “physiological” so to speak, which are inherent in itsvery structure, i.e. a considerable asymmetry, revealed in a sectionalview, and great difficulty in guaranteeing the correct characteristicsof levelness and resistance in the case of high grammages (typicallyover 2000 g/m²) of the product.

In other words, the evident and inevitable asymmetry of thermoformedsheets or films has repercussions on the end product, causing a seriesof well-known drawbacks, whereas the difficulty in producing so-called“heavy” sheets or panels restricts and limits their use in applicationsfor which they are conceptually ideal.

The above asymmetry can first of all be noted in a cooling process ofthe two sides of the thermoformed sheet, which follow two differenttimeframes due to the different mass involved: the thermoformed side, orblister, in fact, due to the same thermoforming process, becomesthinner, losing mass with respect to the bottom and requiring less timefor cooling.

As a result, after the subsequent welding of the thermoformed sheet withthe two outer sheets, the different elastic shrinkage of the two sidesof the thermoformed sheet, specifically due to the different massesinvolved, causes a kind of curling effect, i.e. a transversal curvature,in the end product which, in fact, makes the sheet or panel unusable.

Various procedures, methods and apparatuses have been studied anddeveloped, which have proposed to correct this defect, but the problemof the natural asymmetry of the end product basically remains, and isalso manifested in the different finish of the two outer sides of thesheet, i.e. the outer side welded to the blister side of thethermoformed sheet often has the imprint of the blisters themselves,consequently making the final panel or sheet unusable for high-qualityapplications.

A partial solution to this latter problem is described in patentapplication MI2014A001110: this describes a honeycomb sheet or panelbased on thermoplastic polypropylene, comprising a structure consistingof two flat outer films, thermowelded to a central film consisting of athermoformed blister film repeated in a regular and continuous pattern,the two flat outer films being composed of a coextruded two-layer film(AB), based on thermoplastic polypropylene, wherein the internal layer(A) faces the central thermoformed film, and the central thermoformedblister film is composed of a coextruded three-layer film (ABA), basedon thermoplastic polypropylene, wherein the two outer layers (A) facethe flat outer films and said structure also envisages a further layer Cthermowelded to the outer layer B of the two flat outer films.

The process and apparatus that represent the state of the art closest tothe present invention are described in EP1638770. The process describedin EP'770 envisages the production of a blister sheet with the followingpassages: extrusion of an upper or bottom film, a central film and alower or top film, starting from the corresponding granule;thermoforming of the central film; calibration and partial cooling ofthe bottom and top film; heating of at least one side of the bottom andtop film and coupling of the two bottom and top films to thethermoformed film. This process is carried out in an apparatus thatcomprises at least three extrusion heads, which form the extrusiongroup, followed by a thermoforming group and calibration and coolinggroups, the thermoforming group and calibration and cooling groups inturn being connected to a coupling group. This process, which alreadyallows a high-quality product to be obtained, does not completely solvethe problems indicated herein, as the central thermoformed sheet in anycase has a basic asymmetry, which creates the necessity of usingadditional stabilization and tempering devices (so-called hot-air andcold-air ovens) downstream of the extrusion and forming processdescribed above.

Furthermore, in the case of sheets or panels having a high grammage anda high thickness, a physical limitation to their production is alsolinked to the thermoformability of the material used, which comprisesnot only the polymer (i.e. the polypropylene mentioned above) but alsomineral fillers such as calcium carbonate, with the double objective ofincreasing the resistance characteristics of the end product andreducing the material cost.

On the one hand, the necessity of having materials with an extremelyhigh elastic modulus, i.e. that require considerable force for causing asmall elongation, limits the possibilities of effecting “deep”thermoformings, i.e. thermoformings necessary for the production ofsheets having a high thickness and grammage.

A further aspect to be taken into consideration is the necessity of“charging” the polymer with mineral fillers in different percentages,typically from 10% to 60% by weight, which creates a further limitation,i.e. the necessity of using so-called “compounds”, i.e. compounds ofmineral fillers and polymer (in this case polypropylene), previouslymixed and extruded in the form of granules. This process is currentlyeffected off-line by granulating machines.

Furthermore, in the case of the use of compound granules, the producerof sheets, i.e. of the end product, is compelled to avail of anextremely large stock of raw materials: he must in fact have at hisdisposal granules with various concentrations of material charged fordifferent uses, consequently a necessity that requires the availabilityof numerous, different lots of compounds, each characterized by adifferent concentration of the filler and/or a different mineral filler.This also affects the sheet producer with respect to the suppliers ofthese compounds: he will generally be compelled to have a limited choiceof both suppliers and compounds with different concentration percentagesof mineral fillers, of the same supplier.

The present invention proposes to provide a sandwich sheet or panel madeof thermoplastic material, which overcomes the drawbacks of the knownart.

More specifically, the objective of the present invention is to providea honeycomb sandwich sheet or panel, based on thermoplasticpolypropylene, composed of outer films (or sides), joined to a centrallayer (or core) which comprises thermoformed blister films having acylindrical form or other forms (honeycomb structure) repeated in aregular and continuous pattern, wherein the particular conformation ofthe central layer gives the sheet mechanical properties, load resistanceand structural and functional characteristics.

An objective of the present invention is therefore to provide ahoneycomb sandwich sheet or panel, based on polypropylene, that has thespecific characteristics of the sheet object of patent applicationMI2014A001110, at the same time eliminating the drawbacks previouslydescribed.

An objective of the present invention is therefore to provide ahoneycomb sandwich sheet or panel, characterized by a much greaterthermoforming depth with respect to the thermoformed sheets or panelsaccording to the state of the art, typically a double forming depth.

A further objective of the present invention is to provide a honeycombsandwich sheet or panel with a complete symmetry of the structure of thethermoformed blister sheet or panel.

Finally, objectives of the present invention are a process and apparatusfor producing the honeycomb sandwich sheet or panel that also allow theuse of mineral fillers directly in powder form.

An object of the present invention relates to a honeycomb sandwich sheetor panel, based on thermoplastic polypropylene, comprising a structureconsisting of two flat outer films, at the top and bottom, welded to atleast two inner or central thermoformed blister films, repeated in aregular and continuous pattern, wherein said at least two innerthermoformed films are welded to each other, said sheet or panel beingcharacterised in that said flat outer and inner thermoformed films arecomposed of three coextruded layers.

The inner thermoformed films are present in an even number, equal to orhigher than two, and are preferably two or four, even more preferablytwo.

The inner thermoformed films can be welded to each other directly orthrough the interpositioning of a flat non-thermoformed film.

The at least two inner thermoformed films can be welded to each other bymeans of a “blister-to-blister” welding, or a “bottom-to-bottom”welding, preferably by means of a “blister-to-blister” welding.

The flat non-thermoformed film that can be interpositioned between thetwo central thermoformed films preferably has the same structure withthree co-extruded films and composition as the flat outer films.

In the following description, unless otherwise specified, the term“honeycomb sheet or panel” refers to a structure composed of flatnon-thermoformed films and thermoformed films welded to each other, theterm “films” refers to the single thermoformed or non-thermoformedelements that form the sheet, the term “layer” refers to the elementsforming the film.

Each film consists of three coextruded layers, whereas each sheet orpanel is composed of at least two non-thermoformed films and at leasttwo thermoformed films extruded contemporaneously.

In particular, the honeycomb or blister sheet or panel according to thepresent invention consists, as already indicated, of a flat upper or topouter film, two (or four, or six, etc.) central thermoformed films and aflat lower or bottom outer film.

Each single film is preferably composed of three layers, for exampleaccording to the structures A-B-A, A-B-C, C-B-A and all combinationsthereof.

It is not necessary for all the layers A, B or C, forming the differentfilms to come from the same extruder (i.e. extruder A, or B, or C), butthe presence of the same number of extruders as the layers forming thewhole of the final sheet can be envisaged. In other words, if the finalsheet has a structure consisting of A-B-A+A′-B′-A′+A″-B″-A″+A″′-B″′-A″′,there can be an extruder A for the layer A, an extruder B for the layerB, an extruder A′ for the layer A′, an extruder B′ for the layer B′, andso forth.

The flat outer films preferably consist of films of polypropylenecopolymer (preferably a block or random polypropylene copolymer, withethylene and/or butane monomers inserted in the propylene chains) andhomopolymer and/or relative mixtures, said films having chemicalproperties that make them particularly suitable for hot coupling, at thesame time ensuring a high adhesion and relatively low weldingtemperatures. This is therefore a multilayer structure produced incoextrusion with three layers, wherein the three-layer coextrudedstructure A-B-A is preferably composed of an internal layer B ofpolypropylene homopolymer and two outer layers A of polypropylenecopolymer.

In the case of coextruded multilayer structures A-B-C or C-B-A orcombinations thereof, the structure is the result of the coextrusion ofthree different polymers coming from three different extruders, the twoouter layers of the film A and C can therefore be composed of differenttypes of polypropylene, whether they be polypropylene homopolymers,copolymers, or of any other nature.

The possibility/necessity of having three different polymers in thefilms that form the flat outer films of the sheet or panel according tothe present invention, depends on the fact that, in some applications,the outermost layer of the film can be coupled with possible furtherouter films: consequently, depending on the structure selected, it canbe the layer A or C, and may have to meet particular physical/chemicalcharacteristics, bearing in mind the requirement that the layer of flatouter film directly welded to the inner or central thermoformed film,must have weldability characteristics suitable for guaranteeing theperfect adhesion between said thermoformed film and non-thermoformedfilm.

These considerations are obviously valid for both of the flat outerfilms.

The inner thermoformed films preferably consist of polypropylenecopolymer (preferably a block or random polypropylene copolymer, withethylene and/or butane monomers inserted in the propylene chains) andhomopolymer and/or relative mixtures, having particularly highphysico-mechanical properties, and therefore suitable for guaranteeingthat the end product, i.e. the honeycomb or blister panel or sheet, hasthe desired qualities from the point of view of mechanical resistanceand lightness. This is therefore a film with multilayer structureproduced in coextrusion with three layers, wherein the three-layercoextruded structure A-B-A is preferably composed of an internal layer Bof polypropylene homopolymer and two outer layers A of polypropylenecopolymer.

The outer layers of the flat outer films and inner thermoformed films,the same as or different to each other, are preferably made ofpolypropylene copolymer and the inner layers of the flat outer films andinner thermoformed films are preferably made of polypropylenehomopolymer, possibly with the addition of mineral fillers.

An object of the present invention also relates to a process for theproduction of a honeycomb sandwich sheet or panel which comprises thefollowing phases:

-   a) contemporaneous extrusion of at least four multilayer films,    starting from a corresponding polymer or from a corresponding    polymer compound and mineral filler: an outer bottom film, at least    two inner or central films, and an outer top film;-   b) thermoforming of said at least two inner or central films;-   c) coupling by means of thermowelding of said non-thermoformed flat    outer films with said two inner or central thermoformed films,    wherein said inner thermoformed films are welded to each other    according to a blister-to-blister or bottom-to-bottom arrangement,    and wherein the extrusion phase of the films and the beginning of    the coupling phase are contemporaneous for all the outer and inner,    thermoformed and non-thermoformed films.

The process for the production of a honeycomb sandwich sheet or panelaccording to the present invention can also comprise, upstream of phasea), a mixing phase of polypropylene and mineral filler in powder form,to form the compound in granule form to be used in the subsequent phasea).

Furthermore, the process that envisages said further mixing phase, canbe carried out in continuous, introducing one or more twin-screwextruders prior to the extrusion head(s), so as to feed the processdirectly with pure polypropylene granules and powder of mineral fillers,such as for example, calcium carbonate, fiberglass or the like.

As already specified, the at least two inner thermoformed films can bewelded to each other with a “blister-to-blister” or “bottom-to-bottom”welding. In this way, a honeycomb sheet or panel is finally obtained,composed of four films, of which two central thermoformed films, whichare absolutely and completely symmetrical.

Analogously, honeycomb sheets or panels can be produced using aplurality, again in an even number, of thermoformed films (2, 4, 6,etc.).

FIG. 1 enclosed represents an example of a honeycomb sheet or panelaccording to an embodiment of the present invention.

An essential aspect of the process according to the present invention isthat the production of this type of sheet or panel takes place incontemporaneous regime conditions: this means that all the elements(i.e. all the films, thermoformed and non-thermoformed) must be producedat the same moment, in order to guarantee the necessary symmetry, notonly geometrical (relatively simple to obtain), but also and above all athermal symmetry.

These elements, in fact, must not only be produced at the same moment,but they must also reach the couplings under substantially similarthermal conditions, specifically to guarantee the thermal symmetry.

The film run (i.e. the path followed by all of the films) is extremelyimportant and has been specifically conceived so that the films reachthe coupling phase with substantially similar characteristics.

The term substantially similar characteristics means that each filmreaches the coupling phase at a temperature ranging from −10° C. to +10°C. with respect to the welding temperature of the material used, or thatthe differences in temperature between the films reaching the couplingphase are lower than 10° C. approximately.

In particular, the thermoforming can be either mechanical or undervacuum and the welding of the four (or six, or eight, etc.) films cantake place according to the four embodiments of the process for theproduction of the panel or sheet according to the present invention,described in the enclosed FIGS. 2-5, according to the followingprocedures:

-   -   welding first a smooth outer film 10 to an inner thermowelded        film 11 and, contemporaneously, a second smooth outer film 13 to        a second inner thermoformed film 12; then welding the two pairs        of films or intermediate elements 10-11 and 12-13 thus obtained,        to each other, blister-to-blister (or bottom-to-bottom,        depending on the requirements of the final application), until        the end product is obtained, i.e. the honeycomb sheet or panel        10-11-12-13 (shown in FIG. 2);    -   welding first the two inner thermoformed films 11 and 12 to each        other, by mechanical action or under vacuum, blister-to-blister        (or bottom-to-bottom depending on the requirements of the final        application), then welding, in a single step, the two smooth        outer films 10 and 13, onto the intermediate element 11-12,        until the end product is obtained, i.e. the honeycomb sheet or        panel 10-11-12-13 (shown in FIG. 3);    -   welding all four (or six, or eight, etc.) films in a single        step, of which the two outermost films 10 and 13, smooth, and        the two inner or central films 11 and 12, thermoformed by        mechanical action or under vacuum, superimposing them        blister-to-blister (or bottom-to-bottom depending on the        requirements of the final application), until the end product is        obtained, i.e. the honeycomb sheet or panel 10-11-12-13 (shown        in FIG. 4).

The coupling phase by means of thermowelding c) described above is theneffected by welding the films 10,11,12,13 to each other, wherein eachfilm 10,11,12,13 reaches said coupling phase at a temperature rangingfrom −10° C. to +10° C. with respect to the welding temperature of theouter layer of the same film 10,11,12,13 or the films 10,11,12,13 reachthe coupling phase at temperatures which differ from each other by avalue lower than 10° C.

A fundamental advantage, however, of the honeycomb sheet or panelaccording to the present invention is the perfectly symmetricalstructure and a thermoforming depth which is at least double withrespect to a solution with a single thermoformed film, obviously withthe same raw material used (and therefore thermoformabilitycharacteristics).

In all the embodiments described above (FIGS. 2-4), a further flat film14 can be envisaged, positioned between the two inner or centralthermoformed films 11 and 12, which improves and guarantees the perfectadhesion of all the films forming the structure 10,11,12,13 of thehoneycomb sheet or panel according to the present invention (shown inFIG. 5).

Furthermore, the versatility of the process according to the presentinvention also allows sheets or panels that are intentionallyasymmetrical to be obtained, by simply welding the two (or four, or six,etc.) thermoformed films 11 and 12, blister on bottom or viceversa;sheets or panels can therefore be obtained that are interesting howeverfor applications that do not require a complete symmetry, but that inany case require a thermoforming depth which is at least double withrespect to what is currently available with the products of the state ofthe art available on the market.

A further object of the present invention also relates to an apparatusfor the production of a honeycomb or blister sheet or panel, saidapparatus comprising an extrusion group consisting of at least fourextrusion heads, downstream of said extrusion group there being at leasttwo thermoforming groups with relative calibration and cooling groups,the thermoforming groups and the calibration and cooling groups beingconnected, in turn, to one or more coupling groups.

Said apparatus can also comprise one or more twin-screw extrudersupstream of the extrusion group.

A further object of the present invention relates to the use of thehoneycomb blister sheet or panel according to the present invention, asa protection element or packaging element.

As already observed, a fundamental advantage of the honeycomb sheet orpanel according to the present invention is the perfectly symmetricalstructure and a thermoforming depth which is at least double withrespect to a solution with a single thermoformed film.

One of the advantages of the process for the production of the honeycombor blister sheet or panel according to the present invention consists inthe production of the panel by means of a single production step,wherein single production step means that the final sheet or panel isproduced in a single plant starting from the raw material (i.e. thegranule), without the production of semifinished products, with aconsequent lower energy consumption and, indirectly, with a lesserenvironmental impact, above all thanks to the drastic reduction in wasteproducts.

A further advantage is represented by the absence of delamination thanksto the absolute adhesion specifically guaranteed by the fact that eachsingle film forming the sheet or panel according to the presentinvention is composed of three layers contemporaneously extruded bymeans of a coextrusion process.

In this way, another problem present in the products and processesaccording to the state of the art can also be limited, if not completelyeliminated, i.e. the necessity of finding a mixture of materials thathas good physico-mechanical characteristics and at the same time allowsa coupling at relatively low temperatures, without the risk ofdetachment of the films forming the honeycomb blister panel or sheet.

A mixture of materials that satisfies these requirements is inevitablythe result of a compromise between the requirements of the productionprocess and the desired characteristics for the end product, i.e. thehoneycomb sheet or panel, but both of these needs cannot clearly be 100%satisfied, as this is a compromise, as already mentioned.

In particular, as previously described, the honeycomb blister panel orsheet according to the present invention is composed of a flat outerupper or top film, two (or four, or six, etc.) inner or centralthermoformed films and a flat outer lower or bottom film.

For higher grammages, as already indicated, the presence of mineralfillers is necessary, which are suitable for guaranteeing the correctmechanical resistance properties of the honeycomb blister panel orsheet, at the same time reducing the material cost of the end product.

Furthermore, the process for the production of the panel or sheetaccording to the present invention also allows the concentration ofmineral fillers to be varied, without having to stock the correspondingcompound, and above all it allows said fillers to be used in powderform, i.e. in their natural configuration state. This solution isenabled by the choice of a so-called twin-screw extruder for theextrusion of the layer(s) of coextruded films that comprise said mineralfillers.

In the apparatus according to the present invention, a twin-screwextruder can also be envisaged for the extrusion of the layer(s) thatrequire mineral fillers. A single twin-screw extruder can be present,that can feed all the extrusion heads necessary for the production ofthe panel or sheet according to the present invention (typically notless than three, but there can be 4, 5, 6 and so on) with the use of thesame number of gear pumps, or as many twin-screw extruders can bepresent as the layers of film forming the end product of the panel orsheet according to the present invention, that also require the presenceof mineral fillers, coupling each extrusion head with a gear pump toguarantee the correct flow-rate of material also in the presence of highcounter-pressures of the extrusion head, typical of these applications.

Further advantages of the process for the production of the honeycomb orblister panel or sheet according to the present invention are thefollowing: first of all, this is a continuous process that starts fromthe granule and/or even from the polymer and powders of mineral fillersand directly produces the end product, i.e. the honeycomb sheet orpanel, without intermediate passages. A stock of film reels is thereforenot necessary, with relative economical advantages with respect to boththe logistics and transportation.

It is also possible to directly produce any required grammage (obviouslywithin a defined variation range) and with any colouring, practically“just in time”, with a minimum product waste for obtaining the variationin thickness.

The process according to the present invention also allows aconsiderable energy saving deriving from the fact that all of the filmsinvolved in the process itself require little heating, thanks to thesufficiently high caloric content they maintain in close proximity ofthe various couplings.

The process according to the present invention also has the furtheradvantage of envisaging the use of materials having high mechanicalproperties, as central layer of the three-layer coextruded product,which forms both the flat outer films and also the inner thermoformedfilms, without influencing the weldability of the single films.

Materials having high weldability characteristics can also be used asouter layers of the three-layer coextruded product, which forms both theflat outer films and also the inner thermoformed films, withoutinfluencing the mechanical properties of the end product.

Furthermore, the process according to the present invention has thedefinite advantage of minimizing waste products during the startingoperations, as this is a continuous process, and also and above all atregime, thanks to the possibility of recycling the cut edges forpreferably feeding the extruders of the central layers of thethree-layer coextruded products, that form all of the films, withoutsignificant variations in the characteristics of the end product.

The honeycomb or blister panel or sheet according to the presentinvention is also characterized by the complete absence of residualinternal stress, above all in the case of films having a completelysymmetrical structure (i.e. where the thicknesses of the flat outerfilms are substantially equal and the inner thermoformed films aresymmetrically positioned, i.e. blister-to-blister/bottom-to-bottom).

Furthermore, the honeycomb or blister panel or sheet has a highlevelness, also thanks to the coupling of the films, which is obtainedat temperatures close to the vicat temperature and thanks to the use ofspecific materials for the outer layers, which allow a strong adhesionalso in the presence of relatively limited contact pressures, inaddition of course to the fact that the couplings of the flat outerfilms are preferably effected with the bottom of the inner or centralthermoformed films.

Finally, the process according to the present invention allows honeycombsheets or panels with a high grammage and high thickness, typically over2000 g/m², to be produced without any problems, thanks to the presenceof a number of central thermoformed films that have ablister-to-blister/bottom-to-bottom coupling, therefore allowing aconsiderable increase in the total depth of the thermoforming, also inthe presence of materials having a poor thermoformability, such as thoseused in the production in question, but which are necessary forguaranteeing the correct mechanical properties of the end product.

The sheet or panel according to the present invention has a weightranging from 300 g/m² to 5000 g/m², preferably from 1000 g/m² to 4000g/m².

The sheet or panel according to the present invention has a thicknessranging from 4.00 to 40.0 mm, preferably ranging from 10.00 to 30.00 mm.

The blisters present in the central thermoformed layer have a diameterranging from 3.00 to 20.00 mm, preferably ranging from 4 to 15 mm andthe height of the protrusions/blisters varies and depends on thediameter of the same, for example the height being 3.00 mm for adiameter of 3.5 mm and 10.00 mm for a diameter of 15.00 mm.

The layers A, B and C can have the same or different thickness and saidthickness preferably ranges from 100 microns to 2 mm.

A sheet or panel according to the present invention is preferablycomposed of four films, of which two films (A-B-A and A″′-B″′-A″′) areouter films and flat, welded to two films (A′-B′-A′ and A″-B″-A″)thermoformed and welded blister-to-blister.

The sheet or panel represented in FIG. 1, has optimal stratificationpercentages for contemporaneously guaranteeing the correct weldabilitycharacteristics between the various films and possibly with additionalfilms to be coupled with the outermost layers A and A″', and the correctmechanical properties in terms of crush resistance, flexural modulus andacoustic and thermal insulation. More specifically, both the two flatouter coextruded films A-B-A and A″′-B″′-A″′, and also the two centralthermoformed films A′-B′-A′ and A″-B″-C″, have the followingcharacteristics: the outer layers A, A′, A″, A″′ have a thickness, thesame as or different from each other, ranging from 5 to 10% with respectto the total thickness of the corresponding coextruded film, whereas thecentral layers B, B′, B″, B″′ have a thickness, the same as or differentfrom each other, ranging from 80 to 90% with respect to the totalthickness of the corresponding coextruded film.

Furthermore, in the sheet or panel according to the present invention,in the embodiment according to FIG. 1, each flat outer film and eachthermoformed inner film represents about 20-30% by weight with respectto the total weight of the sheet itself.

The flat outer films preferably have the same weight and the innerthermoformed films preferably have the same weight.

A specific example of a sheet or panel according to the presentinvention has the following features:

-   Sheet with a weight of 3000 g/m²;-   Diameter of the blister deriving from the blister-to-blister    coupling of the two inner thermoformed films: 14 mm;-   Composition of the outer films (upper/flat A-B-A and lower/flat    A″′-B″′-A″′):-   Layer A, A″′:-   PP copolymer (fluidity index=3 g/10′);-   Layer B, B″′:-   PP homopolymer (fluidity index=3 g/10′) and mineral fillers in a    quantity equal to 20% by weight.

The inner or central thermoformed films A′-B′-A′ and A″-B″-A″ envisagelayers A′ and A″ with the same composition as the layers A and A″′ andlayers B′ and B″ with the same composition as the layers B and B″′ ofthe outer films indicated above.

The sheet was produced by first welding the smooth outer film A-B-A tothe inner thermoformed film A′-B′-A′ (by means of the vacuum technology)and, contemporaneously, the second smooth outer film A″′-B″′-A″′ to thesecond inner thermoformed film (by means of the vacuum technology)A″-B″-A″; the two pairs of intermediate elements A-B-A/A′-B′-A′ andA″-B″-A″/A″′-B″′-A″′ thus obtained, were then welded to each other,blister-to-blister, until the end product was obtained, i.e. thehoneycomb sheet or panel A-B-A/A′-B′-A′/A″-B″-A″/A″′-B″′-A″′.

Weight distribution of the films with respect to the total weight of thesheet:

Upper film A-B-A/ central film A′-B′-A′/ central film A″-B″-A″/lowerfilm A″′-B″′-A″′=30%-20%-20%-30%.

The panel obtained has a total weight equal to 3000 g/m².

The invention claimed is:
 1. A honeycomb sandwich sheet or panelcomprising: a structure consisting of two flat outer films made frompolypropylene, the two flat outer films being in top and bottompositions, and being welded to at least two inner thermoformed blisterfilms made from polypropylene, and disposed with a repeated, regular,and continuous pattern, wherein said at least two inner thermoformedfilms are welded to each other, and wherein the flat outer and innerthermoformed films are composed of three coextruded layers.
 2. Thehoneycomb sandwich sheet or panel according to claim 1, wherein theinner thermoformed films are present in an even number, equal to orhigher than two.
 3. The honeycomb sandwich sheet or panel according toclaim 1, wherein the inner thermoformed films are welded to each otherdirectly or by interposing a flat non-thermoformed film.
 4. Thehoneycomb sandwich sheet or panel according to claim 1, wherein theinner thermoformed films are welded to each other with a“blister-to-blister” welding, or a “bottom-to-bottom” welding.
 5. Thehoneycomb sandwich sheet or panel according to claim 1, wherein the flatouter and inner thermoformed films are composed of three coextrudedlayers of a polypropylene copolymer and homopolymer or mixtures thereof.6. A process for producing a honeycomb sandwich sheet or panel accordingto claim 1, comprising the following steps: a) contemporaneouslyextruding at least four multilayer films (10,11,12,13), starting from apolymer compound, or a polymer compound and mineral filler, at leastfour multilayer films comprising an outer bottom film (10), at least twoinner or central films (11,12), and an outer top film (13); b)thermoforming said at least two inner or central films (11,12); and c)coupling by thermowelding, said flat outer bottom and top films (10,13)with said two inner or central thermoformed films (11,12), wherein saidtwo inner or central thermoformed films (11,12) are welded to each otheraccording to a blister-to-blister or bottom-to-bottom arrangement, andwherein the step of extruding of the four multilayer films and abeginning of the step of coupling are contemporaneous for all the outerand inner or central, films (10,11,12,13).
 7. The process according toclaim 6, wherein the polymer compound comprises polypropylene, furthercomprising, upstream of step a), a mixing step of the polymer and amineral filler in powder form to produce a granular compound, whereinthe granular thus obtained is fed to said step a).
 8. The processaccording to claim 6, wherein the coupling step by thermowelding c) isperformed by: welding first a smooth outer film (10) to an innerthermowelded film (11) and, contemporaneously, a second smooth outerfilm (13) to a second inner thermoformed film (12) to produce two pairsof films, then welding the two pairs of films (10-11) and (12-13) thusobtained, to each other, blister-to-blister or bottom-to-bottom; weldingfirst the two inner thermoformed films (11 and 12) to each other, bymechanical action or under vacuum, blister-to-blister orbottom-to-bottom, then welding, in a single step, the first and thesecond smooth outer films (10 and 13) onto the two pairs of films(11-12); or welding all the outer and the inner films (10,11,12,13) in asingle step, of which the first and the second outer films (10 and 13)and the two inner films (11 and 12), which are thermoformed bymechanical action or under vacuum, superimposing the two inner filmsblister-to-blister or bottom-to-bottom.
 9. The process according toclaim 6, wherein the coupling step by thermowelding c) is performed bywelding the outer and the inner films (10,11,12,13) to each other, eachfilm (10,11,12,13) reaching the coupling step at a temperature rangingfrom −10° C. to +10° C. with respect to a welding temperature of anouter layer of the same film (10,11,12,13), or the outer and the innerfilms (10,11,12,13) reach the coupling step at temperatures which differfrom each other by a value lower than 10° C.
 10. An apparatus forproducing a honeycomb sandwich sheet or panel according to claim 1, saidapparatus comprising: an extrusion group having at least four extrusionheads; and at least two thermowelding groups downstream of saidextrusion group having calibration and cooling groups, the thermoweldinggroups and the calibration and cooling groups being connected, in turn,to one or more coupling groups.
 11. The apparatus according to claim 10,further comprising one or more twin-screw extruders upstream of theextrusion group.
 12. The process according to claim 6, furthercomprising the step of providing the honeycomb sandwich sheet or panelas a protection element or packaging element.
 13. The honeycomb sandwichsheet or panel according to claim 5, wherein wherein outer layers of theflat outer films and the inner thermoformed films are made from apolypropylene copolymer and inner layers of the flat outer films and theinner thermoformed films are made of a polypropylene homopolymer.